Method for limiting interference between satellite communications systems

ABSTRACT

Boeing achieves satellite diversity by having a large discrimination angle for a MEO constellation of communications satellites to limit interference in the Ku-band with GSO communications systems. Each satellite entering an exclusion zone over a GSO ground station terminates all transmissions to provide EPFD within acceptable limits. The Boeing MEO constellation preferably comprises 20 satellites, 5 in each orbit. The four orbits are inclined at about 57° with respect to the equator. Services include an Integrated Digital Service (IDS) and Backhaul Data Service (BDS) to accommodate the needs of different users.

REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of U.S. ProvisionalPatent Application No. 60/174,873, filed Jan. 7, 2000.

TECHNICAL FIELD

[0002] The present invention relates to telecommunications and,particularly, to operation of a medium earth orbit (MEO) satelliteconstellations for voice, video, and/or data communication, generallybroadband, in the Ku-band of the frequency spectrum. More specifically,the present invention relates to interference mitigation techniques forsharing of spectrum between two competing communications systems.Typically, one system uses one or more satellites positioned in ageosynchronous orbit (GEO or GSO), and the other uses an array ofsatellites orbiting in non-geosynchronous orbits (non-GEO or NGSO).

BACKGROUND ART

[0003] The World Radio Conference in 1997 (WRC-97) allocated NGSOsatellite systems co-primary status with GSO satellite systems incertain frequency bands based on provisional limits on the EffectivePower Flux Density (EPFD) that the NGSO systems produce. Studiescontinue to refine these EPFD limits to allow the GSO and NGSO systemsto operate simultaneously sharing the Ku-band of the frequency spectrum.The interference mitigation technique of the present invention reducesthe peak interference levels for the GSO ground station antenna andallows the NGSO system to meet the proposed limits more easily. Thetechnique of the present invention produces interference that is moretolerable to GSO systems than interference produced by the techniquescurrently planned for use by LEO NGSO satellite systems. Interferencethat does occur with the technique of the present invention is primarily(if not wholly) outside the “short term interference” band of the EPFDdistribution curve. In this “short term interference” band, for example,the interference may be -40 dB below the limit.

[0004] A satellite in a geostationary or geosyncheanous orbit (GSO) ispositioned above the equator at an altitude of about 35,800 km and at aninclination of about 0 degrees. A GSO satellite orbits the earth onceper day in synchronous motion with the revolution or rotation of theEarth. The satellite appears fixed in the sky to an observer on thesurface of the earth. Communicating with a GSO satellite has someobvious advantages in that an earth station antenna can remain pointedin one stationary and fixed direction without the need for activecontrol to maintain pointing at the GSO satellite. A GSO satelliteprovides coverage of only a portion of the earth and cannot cover thePolar regions, which are 90° in arc away from the plane of thesatellite. Additionally, the round-trip time delay between an earthstation and the G.50 satellite is relatively large, which can haveundesirable effects on communications.

[0005] NGSO satellite constellations have been proposed to overcome someof these problems. A constellation of NGSO satellites can providecomplete global coverage including the Polar regions, because such NGSOconstellations can include satellites at inclination angles other than0°. Since NGSO satellites are nominally at a lower orbit altitude, theround-trip time delay will be lower.

[0006] An of the early NGSO orbit constellation proposed for spectrumsharing with GSO satellite systems was a Low Earth Orbit (LEO) satellitesystem of up to 80 satellites. This constellation was called(SkyBridge—USAKUL1). By its nature, a LEO NGSO satellite will spend asignificant amount of its orbit period in-line between a GSO satelliteand a point on the earth where there might be a GSO earth station.(FIG. 1) The result is that some interference mitigation appraoch oravoidance technique is required to avoid interference with operation ofthe existing GSO satellite network. In the approach adopted bySkyBridge, the NGSO satellite turns off only the antenna spot beams thatservice an area that would be in an alignment condition between a GSOsatellite and a GSO earth station. This SkyBridge technique avoidsmain-beam to main-beam interference and limits the interference to theGSO earth station receiver. A disadvantage of the SkyBridge technique isthat satellite antenna spot-beams that are not serving the affected areawould still be operating, and sidelobes from these spot-beams couldcause significant interference to the GSO earth stations, particularlyfor earth stations with large antenna apertures. Such interference wouldbe relatively significant, especially in the “short term interference”band, and would be disruptive to the GSO communication system.

[0007] The Ellipso system proposed a Highly Eccentric Orbit (HEO). Whilethe orbits proposed for SkyBridge is circular, and the satellite isalways at the same altitude above the earth, a HEO orbit would use aneccentric ellipse having the center of the Earth as one foci of theellipse. As a result the satellite altitude varies significantly overits orbit. For orbit stability, a HEO satellite operates at a highinclination, and the satellite only communicates during the highaltitude portion of the orbit. As a result of the typical operatingcharacteristics of the HEO orbit, there is a large discrimination anglebetween the HEO satellite and a GSO satellite. No in-line conditionoccurs (NGSO satellite in-line with a GSO satellite and a GSO earthstation), which significantly reduces the peak interference into any ofthe GSO earth stations. The Ellipso system is more completely describedin U.S. Pat. Nos. 5,669,585; 5,788,187; 5,845,206; and 5,979,832, whichI incorporate by reference.

[0008] Individual satellites of a HEO satellite constellation are onlyoperating during a portion of their orbit, increasing the number ofsatellites required to provide continuous communication coverage. Theadditional satellites impose significantly higher start-up and capitalcosts for the system. A Molniya HEO orbit, for example, requires threesatellites. Each operates for only 8-hours of the orbit to provide24-hour coverage. HEO satellites also transit the Van Allen radiationbelts continuously. The satellite therefore has to be significantlyradiation hardened, making it more costly. The tramsot through thisintense cadiation lowers the expected lifetime for each satellite.Because the satellite altitude is constantly changing during itsoperating period, the service area covered by a typical satellitereflector antenna will also constantly change unless active beampointing and beamwidth control is used. Adding beam pointing andbeamwidth control systems to each satellitie requires a more complex andcostly antenna control system.

[0009] The system Ellipso described in U.S. Pat. No. 5,979,832 involvesan array of satellites that looks like a planetary gear system. Thesatellites are in low to medium earth orbit (LEO to MEO) in twointeractive orbital rings. An outer ring contains circular orbitsatellites. An inner ring contains elliptical orbit satellites. Theapogees of the elliptical orbits are approximately tangential to thediameter of the circular earth orbits. The periods of the two rings areadjusted to be proportional to the numerical ratio of the number ofsatellites in one ring with that of the other. The adjustment allows theelliptical inner ring of satellites to be spaced always midway betweenthe satellites (or “teeth”) of the outer ring for a specified parameter.This spacing can be tailored to a specific point on the earth or to agiven time of day. The spacing between satellites in the “planet gear”constellation will be approximately equal anywhere in the populatedworld during daytime hours. Nighttime coverage is likely less criticalsince fewer people will be using resources at night—more people aresleeping. Hence, the circular satellites are presumed to be capable ofhandling the nighttime traffic alone, without involving the ellipticalsatellites. The fact that the inner elliptic ring of satellites overtakeand pass the outer circular ring of satellites on the nighttime side ofthe earth is thus not a cause for serious concern, provided that theassumption of a decline in load actually occurs. If messages are storedand transmitted prefecentially at “off-peak” hours, the relativeamplitudes of the peaks and troughs in usage, however, would be reducedand load might approach a steady state. This HEO system might encounterproblems with a steady state pattern of use.

[0010] “Apogee pointing toward the sun” (APTS) satellites useful in theEllipso elliptical orbits are described in U.S. Pat. No. 5,582,367,which I also incorporate by reference.

[0011] A Medium Earth Orbit (MEO) constellation of the present inventioncan provide the advantages of the HEO system (large discriminationangles) in reducing the overall interference to the GSO satellitenetworks without the disadvantages of the HEO system. The interferenceto the GSO satellite networks can be significantly reduced over thatproduced by the LEO systems without the inefficiencies (i.e., need for alarger number of satellites), complexity, or cost of the HEO systems.

[0012] A MEO satellite system is described in U.S. Pat. Nos. 5,433,726;5,439,190; 5,551,624; and 5,867,783, which I incorporate by reference.Such a system would be able to provide complete global coverage,including the Polar regions, which the GSO satellite networks cannot do.

[0013] A MEO/GSO satellite system is described in U.S. Pat. No.5,971,324, which I also incorporate by reference.

[0014] U.S. Pat. No. 6,011,951 describes an interference avoidancetechnique for two Teledesic LEO constellations sharing a common radiofrequency band. A first and a second satellite communication system eachcontain a plurality of satellites in a plurality of non-geostationary(NGSO) Earth orbits. Each of the plurality of NGSO satellites has apredefined orbital plane. Within each orbital plane, satellites of thefirst and second satellite communication systems are alternating, suchthat each orbital plane contains one or more satellites from both of thesatellite systems. In this manner, it is possible to achievesatisfactory discrimination between satellites and Earth-based stations.The Earth-based station of each communication system will communicatewith the closest satellite of its respective communication system. In analternative technique that is particularly useful when an Earth-basedstation in the first communication system is able to communicate withmore than one satellite, a satellite is selected based on thetopocentric separation of the satellite from satellites in the secondsystem. The system can also combine alternating satellites within anorbital plane with alternating orbital planes with satellites of eachrespective communication system.

[0015] A need remains for a technique to limit interference between aMEO constellation and a GSO system. The present invention provides sucha technique.

[0016] A need also remains for a low-cost, reliable satellitearchitecture suitable for use in a MEO system to provide globalcommunication coverage to and from mobile platforms, such as airplanes,trains, ships, or automobiles. Sucha a stellite constellation wouldallow access to the Internet, for example, to a global population withincreasing frequency to be “on the move.”

[0017] These and other features of the present invention will now bedescribed in the Summary and Detailed Description.

SUMMARY OF THE INVENTION

[0018] The preferred Boeing MEO satellite constellation communicationsystem includes 20 satellites, preferably at an altitude of about 20,182km. The system consists of four planes inclined 57° relative to theequator, with each plane containing five satellites. The use of a MEOconstellation enables the Boeing system to provide truly global coveragewhile minimizing the number of spacecraft and, as a result, lowering thecosts for Boeing's customers. The constellation permits users outsidethe Tropics to always be in view of at least two operational satellitesabove a 30° elevation angle. Customers within 23° of the equator will bein view of at least two satellites above a 30° elevation at least 73percent of the time. In addition, each satellite will always be visibleto at least two gateways (i.e., an NG50 Earth station) of the Boeingsystem. The number of satellites is a design choice in large measure,but designs usually seek to minimize the number of satellites becausethey are expensive to build, launch and maintain. The Boeing system isdesigned to provide “bandwidth on demand” (“BOD”) communication and dataservices. To accommodate the unique data transmission needs ofprofessional, institutional and governmental users, the system includestwo types of transmission schemes: Integrated Digital Service (“IDS”)and Backhaul Data Service (“BDS”). The Boeing system may provideancillary broadband communication services to user terminals affixed tomobile platforms, such as aircraft, ships, or motor vehicles.

[0019] To ensure seamless handoffs between gateways, each Boeingsatellite will have two sets of feeder link antennas, receivers andtransmitters. Feeder link antennas will independently track differentgateway locations. Approximately twelve gateways will be used around theworld to provide connectivity between the Boeing system and theterrestrial communications infrastructure.

[0020] The Boeing system employs an interference mitigation techniquethat eliminates main-beam to main-beam interference. Neither thesatellites nor associated earth stations will transmit when the Boeingsatellites are within 15° latitude of the equator. When the Boeingsatellites enter the exclusion zone, traffic is switched to a Boeingsatellite that is not within the exclusion zone.

[0021] The spacecraft antennas include two feeder link antennas, sevenforward service link antennas and one return service link antenna forthe IDS; along with transmit and receive multi-beam phased arrayantennas for the BDS and an Earth-coverage beacon antenna.

[0022] The flexibility and reliability of Boeing's dual BOD transmissionschemes will be able to accommodate a wide variety of professional,institutional and governmental uses. A sampling of potentialapplications include:

[0023] Corporate networking for dispersed corporate offices where anumber of people in individual in-house networks are tied together in awide area mesh network.

[0024] Banking and commercial transactions, where documents, contractsand databases need to be exchanged with substantial accuracy and in asecure mode.

[0025] Distance learning for corporate training, specialized educationand professional seminars.

[0026] Medical applications include the exchange of data, X-ray images,CAT scan data and EKG traces.

[0027] Publishing, where designers, artists and customers must exchangehigh-resolution color images—both fixed and moving.

[0028] Entertainment, where high-resolution audio and video materialmust be backhauled to a central production and redistribution facility.

[0029] Remote mining and exploration activities, where geologicalsampling data needs to be transmitted back to a central location foranalysis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a schematic showing typical interference from an NGSOsatellite flying over a GSO earth station.

[0031]FIG. 2 is a schematic showing a preferred interference avoidancetechnique of the present invention.

[0032]FIG. 3 is a plot showing the anticipated performance (% timeexceeded v. EPFD) of a system implementing the technique shown in FIG.2.

[0033]FIG. 4 is a plot showing the anticipated performance (% timeexceeded v. EPFD) for a system, such as the SkyBridge system, not usingthe technique shown in FIG. 2.

[0034]FIG. 5 is a plot showing the discrimination angle as a function ofthe assumed latitude of the GSO earth station.

[0035]FIG. 6 is a plot showing sidelobe discrimination pattern as afunction of angle for a reference antenna.

[0036]FIG. 7 shows another sidelobe discrimination pattern, similar toFIG. 6, for a reference antenna.

[0037]FIG. 8 shows yet another sidelobe discrimination pattern foranother reference antenna.

[0038]FIG. 9 is a plot of power flux density as a function of elevationangle.

[0039]FIG. 10 is a plot of the “worst case” EPFD for a referenceantenna.

[0040]FIG. 11 shows the satellite handoff process.

[0041]FIG. 12 is a typical frequency band plan.

[0042]FIG. 13 is a block diagram of the IDS feeder link.

[0043]FIG. 14 is a block diagram of the IDS return link.

[0044]FIG. 15 is a block diagram of the BDS feeder link.

[0045]FIG. 16 is a block diagram of the BDS return link.

[0046]FIG. 17 illustrates Boeing's preferred constellation.

DETAILED DESCRIPTION

[0047] 1. Interference Avoidance

[0048] In a preferred embodiment, an NGSO satellite 12 (FIG. 2) operatesat an altitude of about 20,000 km. Obviously, a satellite at lower orhigher altitudes could also work with the inventive method, but MEO isthe preferred altitudes with a lower practical limit likely of about10,000 km. LEO places the satellites too close to the earth. Theexclusion angle becomes large to achieve satellite diversity (as much as80%) so the constellation requires an impractical, large number ofsatellites. The interference avoidance technique of the presentinvention involves satellite diversity. To minimize the interference toa GSO satellite system consisting of a GSO earth station 10 and a GSOsatellite 13, the NGSO satellite 12 turns off all transmissions when itenters an exclusion zone 14 around the equator. In the presentimplementation of the invention, the exclusion zone is ±15 degreeslatitude. Coverage of areas within the exclusion zone is accomplished byhandling of the communication to another NGSO satellite outside of theexclusion zone.

[0049] Defining an exclusion zone of ±15° latitude permits a practicalsystem with a relatively small number of satellites. Boeing'sconstellation uses 5 or 6 satellites in each of four inclined orbits toprovide worldwide communications coverage. If the exclusion angle wereadjusted to ±5° latitude, larger ground station antennas would berequired, and we believe they would be impractical, especially for beingcarried on mobile platforms. At an exclusion zone of ±10° latitude, areasonable, practical system can be designed. At exclusion angles higherthan 15°, the discrimination angle increases, but the spot coverage forthe satellite decreases, thereby forcing the use of more satellites. Wedo not believe that there is an advantage to defining an exclusion zonewith an exclusion angle greater than ±15° latitude, unless EPFD limitsare tightened further to force greater discrimination between thecompeting, frequency sharing NGSO and GSO systems.

[0050] GSO receiving antennas for ground stations are improving theirdirectional characteristics so that their receiving performance istightening and improving. Therefore, the interference limits are likelyto relax rather than tighten in the future. An optimized NGSO systemwill operate at the EPFD limits (or slightly below) for acceptableinterference because doing so will minimize total system costs.

[0051] There are four interference scenarios that must be considered todetermine the overall interference levels NGSO satellite 12 produce orintroduce into a GSO earth station 10:

[0052] interference from the main-beam of the NGSO satellite 12 antennasinto the main-beam of the GSO earth station 10 receiver (main-beam tomain-beam interference),

[0053] interference from the main-beam of the NGSO satellite 12 antennasinto the sidelobe of the GSO earth station 10 receiver (main-beam tosidelobe interference),

[0054] interference from the sidelobe of the NGSO satellite 12 antennasinto the main-beam of the GSO earth station 10 receiver (sidelobe tomain-beam interference) and

[0055] interference from the sidelobe of the NGSO satellite 12 antennasinto the sidelobe of the GSO earth station 10 receiver (sidelobe tosidelobe interference).

[0056] By not transmitting when in the exclusion zone 14, the NGSOsatellite 12 never operates in the GSO earth station 10 antenna mainbeam to avoid the conditions of main-beam to main-beam interference andsidelobe to mainbeam interference. While a LEO satellite system avoidsmain-beam to main-beam interference, it does have sidelobe to main-beaminterference. It is the sidelobe to main-beam interference that canproduce high peak interference powers to the GSO earth station 10receiver, particularly for large GSO earth station 10 antennas. TheSkyBridge interference avoidance technique allows sidelobe to main-beaminterference. The present invention avoids these large peak interferencepowers by avoiding the sidelobe to main-beam interference condition.

[0057]FIG. 3 shows a plot of the percentage of time a given EPFD levelis exceeded for Boeing's proposed NGSO system using the interferencemitigation approach of the present invention. This plot is a result of asimulation performed for the interference from an NGSO satelliteconstellation into a GSO earth station with a 10-meter diameter antenna.FIG. 4 is a similar plot for the SkyBridge LEO satellite system whichdoes not use the Boeing's interference mitigation approach. As can beseen the interference produced into large GSO earth station antennas ismuch lower for the Boeing approach, making it more acceptable to thecurrent GSO satellite operators.

[0058] The Boeing system employs an interference mitigation approachthat essentially eliminates main-beam to main-beam interference. TheBoeing system satellites preferably are in circular oribits at a MEOaltitude of about 20,182 kilometers. Neither the satellites norassociated NGSO earth stations or gateways 15 (FIG. 1) will transmitwhen the satellites are within 15° latitude of the equator. When theBoeing satellites enter the exclusion zone, traffic is switched to aBoeing satellite that is not within the exclusion zone.

[0059] The earth station has a minimum discrimination angle of at least10° between GSO satellites and all Boeing NGSO satellites—illustratingthe fact that main-beam to main-beam events never occur with the Boeingsystem. The earth station's location—about 76.3° —in our analysis wasselected because it represents the “worst case” location for a GSO earthstation operating pursuant to the Commission's 5° elevation anglerequirement.

[0060] The worst case geometry occurs when an earth station, a GSOsatellite and a NGSO satellite all operate at the same longitude. Thiswa the basic conservative geometric configuration used throughout ourinterference analysis. With the Boeing NGSO satellite at the edge of theexclusion zone (15° latitude), and the GSO earth station at 76.°latitude operating with a minimum elevation angle of 5°, thediscrimination angle between the GSO satellite and the Boeing NGSOsatellite is 10°. Furthermore, a GSO earth station at a higher latitudethat is operating at a lower elevation angle than 5° would still neverbe subject to main-beam to main-beam events with Boeing's preferredarrangement of NGSO satellites.

[0061] In all other realistic scenarios, an earth station communicatingwith a GSO satellite will have a larger discrimination angle between theGSO arc and any operating Boeing satellite. There is no point on Earthwhere a GSO earth station antenna would have a transmitting Boeingsystem satellite in its main-beam and no point on Earth where a Boeingsystem NGSO earth station antenna would have a GSO satellite in its mainbeam. In other words, because of Boeing's use of a MEO constellation anda 15° latitude equatorial exclusion zone, ‘main-beam to main-beam’ and‘sidelobe to main-beam’ interference scenarios do not occur with respectto space-to-Earth transmissions from the Boeing system into GSO earthstations. At the same time, the ‘main-beam to main-beam’ and ‘main-beamto sidelobe’ interference scenarios do not occur with respect toEarth-to-space transmissions from Boeing earth stations into GSOsatellites.

[0062] In WRC-97 Resolutions 130 and 538, provisional limits wereprovided for NGSO transmissions in the space-to-Earth direction toprotect GSO networks. One of the limits is stated as an Effective PowerFlux Density (EPFD), which is defined as:${EPFD} = {10 \cdot {{Log}\left\lbrack {\sum\limits_{i = 1}^{N}{10^{{pfd}_{t}/10} \cdot \frac{G_{r}\left( \theta_{i} \right)}{G_{\max}}}} \right\rbrack}}$

[0063] The provisional EPFD long-term and short-term limits included inResolutions 130 and 538 are provided in Table 1. TABLE 1 EPED Limitsfrom Resolutions 130 and 538 EPFD Time Frequency DBW/m²- Per- Band 4kHzcentage Reference Antenna FSS Bands −179 99.7 60 cm, Rec. 465-510.7-11.7 GHz −192 99.9 3 m, Rec. 465-5 11.7-12.2 GHz −186 99.97 3 m,Rec. 465-5 in Region 2 −195 99.97 10 m Rec. 465-5 12.2-12.5 GHz −17099.999 60 cm, Rec. 465-5 in Region 3 −173 99.999 3 m, Rec. 465-512.5-12.75 GHz −178 99.999 10 m, Rec. 465-5 in Regions 1 & 3 −170 100≧60 cm, Rec. 465-5 BSS Bands −172.3 99.7 30 cm, BO-1213 11.7-12.5 GHz−183.3 99.7 60 cm, BO-1213 in Region 1 −186.8 99.7 90 cm, BO-121311.7-12.2 GHz & −169.3 100 30 cm, BO-1213 12.5-12.75 GHz −170.3 100 60cm, BO-1213 in Region 3 90 cm, BO-1213 BSS Bands −174.3 99.7 45 cm, App.30 Annex 5 12.2-12.7 GHz −186.3 99.7 1 m, App. 30 Annex 5 in Region 2−187.9 99.7 1.2 m, App. 30 Annex 5 −191.4 99.7 1.8 m, App. 30 Annex 5−165.3 100 45 cm, App. 30 Annex 5 −170.3 100 1 m, App. 30 Annex 5 1.2 m,App. 30 Annex 5 1.8 m, App. 30 Annex 5

[0064] To determine the EPFD for a NGSO system, the following variablesmust be provided: (1) the discrimination angle of the GSO earth stationreceive antenna, (2) the sidelobe reference pattern of the GSO earthstation receive antenna and (3) the power fluz density (PFD) from theNGSO satellite.

[0065] The first factor in determining the EPFD for a NGSO system is thediscrimination angle of the GSO earth station receive antenna. Thisdiscrimination angle is the angle from the boresight of the GSO antenna(pointed toward the GSO satellite) to the direction of the interferingBoeing system satellite. FIG. 5 shows a plot of the discrimination angleas a function of the assumed latitude of the GSO earth station. Thisplot assumes that the GSO earth station, the GSO satellite and theBoeing satellite are at the same longitude and that the Boeing satelliteis at a latitude of 15° (at the edge of the exclusion zone). As theBoeing system satellite moves further north, away from the exclusionzone, the discrimination angle will increase and the EPFD at the GSOearth station will decrease.

[0066] The second factor in computing the EPFD for a NGSO system is thesidelobe reference pattern for the receive antenna of the GSO earthstation. As indicated in Resolution 130, the reference pattern used forthe FSS bands is in ITU-R Recommendation 465-5. FIG. 6 is a plot of thesidelobe discrimination as a function of angle for the three antennasizes included in Resolution 130. FIG. 7 shows the sidelobediscrimination pattern for the reference BSS antennas of ITU-RRecommendation BO-1213. FIG. 8 shows the sidelobe discrimination patternfor the reference BSS antenna patterns in Radio Regulation Appendix S30,Annex 5 used for Region 2.

[0067] The final factor in determining the EPFD for a NGSO system is theactual PFD from the NGSO satellites. Boeing is providing two differentdata transmission services to users, IDS and BDS. The two services havedifferent space-to-Earth transmission characteristics and, as a result,produce different PFD and EPFD levels. Accordingly, the PFD and EPFDlevels for each service are considered separately.

[0068] Each Boeing satellite will be capable of producing 37 spot beamsfor IDS space-to-Earth links. Each of the beams will have a differentEIRP and different pointing angle. Consequently, each beam will producea different PFD. FIG. 9 is a composite plot of the PFD produced by eachof the co-frequency beams from the Boeing satellite as a function ofelevation angle. As adjacent beams use different frequency bands, thecomposite shows the maximum value of a single sub-band across all thebeams.

[0069] Spot beams from the Boeing system constellation are controlled sothat under normal conditions a spot beam will serve a ground area fromonly one satellite. Where the co-frequency beams of two or moresatellites completely overlap, the spot beam from only one of thesatellites will normally be operational. The general metric fordetermining which spot-beams will be on or off is based on whichprovides the best (i.e., the highest) elevation angle. The use of ahigher elevation angle will also minimize interference into othersystems.

[0070] At the edges of coverage between two satellites, it is possibleto have two spot beams of the same frequency with partial overlappingfootprints. At the worst case, with two co-frequency spot beamscompletely overlapping, the PFD would be 3 dB higher than a single spotbeam for the duration of the overlap at the particular ground location.Only one satellite at a time, however, will be anywhere near the worstcase geometry. The second satellite with the overlapping spot beam willbe at a significantly greater discrimination angle and, as a result, itwill not have a significant impact on the EPFD level.

[0071] The three variables—satellite PFD, discrimination angle, andreference antenna pattern—can be combined to determine an unrealistic,worst case EPFD that theoretically might be produced by Boeing's IDStransmissions. FIG. 10 is a plot of this unrealistic, worst case EPFDproduced by a single Boeing satellite for the ITU-R Recommendation465-5, 60-cm antenna. The EPFD for a Boeing satellite at the edge of theexclusion zone is plotted against the latitude of the GSO earth station.For reference, the long-term provisional limit is also plotted. A Boeingsatellite providing IDS in Boeing's preferred system will meet or bebelow the provisional long-term limit for the Rec. 465-5, 60-cm antenna.Since the provisional short-term limits include a higher EPFD value thanthe long-term limits, satisfying the long-term EPFD limit 100 percent ofthe time, by definition, satisfies the short-term EPFD limit.

[0072] This “worst case” will not occur under any normal operatingcondition. The worst case condition is unrealistic because it assumesthat all the spot beams for a Boeing satellite are turned on and that noother satellite in the constellation provides coverage of any area beingcovered by the single satellite. Under normal conditions the area of thehigh latitude test point at which the worst case EPFD occurs would notbe served by such a satellite in the worst case geometry. Instead,another satellite in the constellation at a much higher elevation angle(and thus at a higher discrimination angle) would serve the highlatitude test point, resulting in a much lower EPFD. Therefore theBoeing system will meet the requirements to avoid interference with theGSO system during all normal operating scenarios.

[0073] To determine the expected EPFD under normal operating conditions,simulations of the interference conditions between the operationalBoeing system and a reference GSO system have been conducted. Thesimulations were done using the Visualyse software package. A list ofthe pertinent simulation parameters is provided in Table 2. A number oflocations for GSO earth stations were tested to determine the worst caselocation. As expected, the worst case location is where the GSO earthstation is at the same longitude as the GSO satellite, and the NGSOsatellite with a descending node that is just slightly east of that samelongitude. TABLE 2 Simulation Parameters GSO Satellite Location 97° WWorst-case Test Point 97° W, 50° N Location NGSO Satellite Locations Asdefined in Table 6.2-1 NGSO Terminal Location 97° W, 47° N SimulationDuration 24 hours (orbit repeat period) Simulation Step-size 15 seconds

[0074] Table 3 is a summary of the results of the simulations. The Tablealso lists the provisional long-term limit for each antennaconfiguration in Resolutions 130 and 538. It also provides the maximumvalue of the EPFD that was determined from the simulations. The EPFDproduced by the Boeing system is below the provisional long-term limitin all cases. Accordingly, the Boeing system also meets the lessrestrictive short-term limit 100 percent of the time. TABLE 3 EPFDSimulation Results Limit Simulation (Long- EPFD Band Reference AntennaTerm) (Maximum) FSS 60 cm, Rec. 465-5 −179 −184.3 3 m, Rec. 465-5 −192−198.3 10 m, Rec. 465-5 −195 −208.8 BSS 30 cm, BO.1213 −172.3 −181.6 60cm, BO.1213 −183.8 −187.6 90 cm, BO.1213 −186.8 −191.1 BSS 45 cm, App 30Annex 5 −174.3 −180.3 1.0 m, App 30 Annex 5 −186.3 −188.9 1.2 m, App 30Annex 5 −187.9 −190.9 1.8 m, App 30 Annex 5 −191.4 −192.5

[0075] The Boeing BDS uses the same exclusion zone as the IDS toeliminate main-beam to main-beam interference. The BDS uses operationalconstraints to mitigate other forms of interference. The BDS steerablebeams are controlled so that co-frequency beams of two or moresatellites do not overlap on any area of the Earth.

[0076] The results described and shown are representative of the system(simulated) performance as of Jan. 8, 1999. Refinements in the systemand changes to the limits since that time have further improved thesystem. The data presented is representative of the system capabilitiesrather than necessary attributes of the present invention.

[0077] The announced Teledesic system will always point the earthstation antennas North (South) and will locate all the earth stationsoutside and exclusion zone in latitudes between 18° South and 18° North.This technique also will result in a low interference level into the GSOearth stations, but does not provide truly global coverage.

[0078] 2. Boeing's Ku-band NGSO FSS System Design

[0079] Having presented a detailed description of our preferredinterference avoidance technique for Boeing's Ku-band NGSO FixedSatellite Service (FSS) system, I will next provide a generaldescription of the overall system. Further, details are included inBoeing's Ku-band NGSO FSS System Application (for license) to theFederal Communications Commission submitted Jan. 8, 1999, which Iincorporate by reference.

[0080] The preferred Boeing system includes 20 operational satellites ina MEO constellation, preferably at an altitude of about 20,182 km. Thesystem consists of four planes inclined 57° relative to the equator,with each plane containing five operational satellites (and usually onespare). The use of a MEO constellation enables the Boeing system toprovide truly global coverage while minimizing the number of spacecraftand, as a result, lowering the costs for Boeing's customers. Theconstellation permits users outside the Tropics to always be in view ofat least two operational satellites above a 30° elevation angle.Customers within 23° of the equator will be in view of at least twosatellites above a 30° elevation at least 73 percent of the time. Inaddition, each satellite will always be visible to at least two gatewaysof the Boeing system. The number of satellites is a design choice inlarge measure, but designs usually seek to minimize the number ofsatellites because they are expensive to build, launch and maintain.

[0081] The system will operate within two GHz bands in the Ku-band forEarth-to-space transmissions and within the one band for space-to-Earthtransmissions, requiring about 326 MHz of Earth-to-space service linkspectrum and 1000 MHz of space-to-Earth service link spectrum. TheBoeing system also requires 600 MHz of spectrum for Earth-to-spacefeeder link operations and 1000 MHz of spectrum for space-to-Earthfeeder link operations.

[0082] The Boeing system is designed to provide “bandwidth on demand”(“BOD”) communication and data services, primarily to corporate,institutional, governmental and large professional users. Theprofessional BOD market is by far the most rapidly growing segment ofthe commercial satellite services industry. Expansion in the BOD marketis being fueled by the convergence of transmission paths for multipletelecommunications services that were previously provided separately toconsumers. For example, a BOD satellite network can provide toprofessional users a single data stream that can include basic voicetelephony, facsimile transmission, video-conferencing, high-speed datatransfer for local area networks (“LANs”), Internet access, databroadcasts and other services. By utilizing a single data transmissionpath for multiple telecommunications services, professional users cansignificantly lower the cost of information systems, while enjoying thegeographic flexibility that can be provided by a global satellitenetwork.

[0083] To accommodate the unique data transmission needs ofprofessional, institutional and governmental users, the system includestwo types of transmission schemes: Integrated Digital Service (“IDS”)and Backhaul Data Service (“BDS”). IDS is the primary transmissionscheme. Resembling the structure of an Internet service, IDS provideslarger capacity in the forward direction (up to 31.2 Mb/s) and smallercapacity in the return direction (up to 7.37 Mb/s) per beam. Boeing'ssecond service, BDS, is designed primarily to retrieve large amounts ofdata from remote locations. BDS offers high-speed (up to 120 Mb/s)return link transfer capability and permits modest (24.4 Mb/s) forwardlink capability.

[0084] The Boeing system provides two types of data transmission systemsto users in much of the same spectrum. Boeing's BDS forward servicelinks will operate in the same spectrum as its IDS forward servicelinks. Similarly, Boeing's BDS return feeder links will operate in thesame spectrum as its IDS return feeder links. Thus, Boeing will be ableto provide multiple data transmission services on a spectrum efficientbasis, while serving the customized needs of commercial, governmental,institutional and large professional users.

[0085] The Boeing system may provide ancillary broadband communicationservices to user terminals affixed to mobile platforms, such asaircraft, ships, and motor vehicles. A substantial, currentlyunderserved demand exists for broadband data services aboard mobileplatforms. Boeing has developed advanced antenna technology that canprovide service to this market without resulting in unacceptableinterference to authorized users of the Ku-band.

[0086] Boeing's system will also be able to operate co-frequency with FSnetworks in the Ku-band. When the footprints from two Boeing satellitesoperating at the same frequency completely overlap, one beam will beshut down to limit the power flux density (“PFD”) produced at theEarth's surface. As a result, only one transmit beam from the Boeingsystem will be covering the ground in most cases. Even when the beams oftwo satellites partially overlap, the PFD level at that location willstill be below the provisional PFD limits adopted by WRC-97.

[0087] Finally, Boeing's system will be able to operate co-frequencywith other adequately designed NGSO FSS systems. Boeing's system canshare spectrum with multiple homogeneous satellite systems. Boeing'ssystem can also share spectrum with a number of inhomogeneous systems ifappropriate interference mitigation techniques are adopted by eachco-frequency system. For example, satellite diversity could be employedto avoid incidents of ‘main-beam to main-beam’ interference betweensatellites in different inhomogeneous NGSO constellations.

[0088] Boeing's ability to share spectrum with other Ku-band systems isenhanced by Boeing's choice of a MEO constellation. Boeing's MEOconstellation provides superior spectrum sharing capabilities ascompared to comparable low Earth orbit (“LEO”) systems. The spectrumsharing capabilities of Boeing's satellite system is one of many designcharacteristics that make the Boeing system a highly efficient and costeffective use of scarce spectrum resources.

[0089] The use of the preferred MEO constellation will enable Boeing toprovide truly global coverage, while limiting the number of spacecraft.The constellation consists of four planes, each containing fivesatellites. Each orbital plane is inclined 57° relative to the equator.The constellation permits any user of the Boeing system at latitudesabove 23° to always be in view of at least two operational satellitesabove 30° elevation. Boeing customers between the Tropics will be inview of at least two satellites at least 73 percent of the time.

[0090] To ensure seamless handoffs between gateways, each Boeingsatellite will have two sets of feeder link antennas, receivers andtransmitters. Feeder link antennas will independently track differentgateway locations. Approximately twelve gateways will be used around theworld to provide connectivity between the Boeing system and theterrestrial communications infrastructure. Gateways will bestrategically located so that every Boeing satellite will always be inview of at least two gateways. Additional considerations in the sitingof Boeing gateways will include providing a high level of connectivitywith terrestrial telecommunications networks and ease of spectrumcoordination with co-frequency terrestrial services.

[0091] Boeing's service links are designed to communicate with userterminals using dual-circular polarization. Boeing's feeder linktransmissions will employ a combination of single polarization anddual-circular polarization transmissions.

[0092] 2.1 Data Transmission Services

[0093] IDS and BDS are structured to operate in a complementary fashion,sharing the same spectrum for forward service links and return feederlinks. Boeing's BDS forward service links will operate co-frequency withIDS service links using geographic diversity to avoid intra-systeminterference. ‘Geographical diversity’ means that, using electronicallysteerable transmit antennas, each Boeing satellite will provide BDStransmissions only to users that are located in portions of thesatellite's footprint where the IDS service is being provided by anothersatellite.

[0094] Boeing's BDS feeder links will operate cooperatively with the IDSfeeder links. Each Boeing satellite will have two sets of feeder linktransmitters and receivers to perform seamless handoffs betweenterrestrial gateways. IDS gateway handoffs will operate on a“make-before-break” basis, while BDS handoffs will function using a“break-before-make” method. As shown in FIG. 11, during the handoffprocess, both sets of feeder link transmitters and receivers will beused cooperatively to execute the handoff between gateways. During theperiod between gateway handoffs, however, only one set of feeder linktransmitters and receivers will be used for the IDS feeder links, makingthe second set of transmitters and receivers available for BDS feederlink operations.

[0095] The flexibility and reliability of Boeing's dual transmissionschemes will be able to accommodate the customized needs of a widevariety of professional, institutional and governmental users.

[0096] 2.2 Integrated Digital Services (IDS)

[0097] To provide IDS each Boeing satellite generates a pattern of 37spot beams to communicate with user terminals. A three-cell frequencyreuse pattern is used within the 37-cell spot beam array and dualpolarization is used within each spot beam. As a result, Boeing's IDScan have a spectrum reuse factor in excess of 24 within each satellitefootprint and in excess of 493 over the full 20-satellite constellation.

[0098] The IDS transmit and receive antennas on each satellite useidentical spot beam coverage patterns. The IDS beam pattern is nadirpointed and, as a result, the area covered by each satellite beam movesacross the Earth as the satellite progresses through its orbit. Due toBoeing's choice of a MEO constellation, the motion of each satelliterelative to the Earth is comparatively slow and an IDS user terminal cangenerally remain in the same beam for about 27 minutes. Beam handoffsare accomplished through ground control. IDS beams from one Boeingsatellite that completely overlap IDS beams from another Boeingsatellite will be turned off to conserve power and to minimize emissionsinto co-frequency systems.

[0099] The Boeing system will assign user terminals to IDS satellitebeams primarily by using whichever beam provides the highest elevationangle. By giving priority to satellite beams at higher elevation angles,Boeing can ensure data transmission quality and minimize emissions intoco-frequency FS and GSO FSS/BSS services. Boeing anticipates that aftera short period of operation, its user traffic patterns will becomepredictable, enabling Boeing to predetermine when beams of eachsatellite should operate at peak power levels to provide the bestservice in each coverage area.

[0100] Boeing's IDS power controlled forward feeder links will use afrequency division multiplex/time division multiplex (“FDM/TDM”) signal,with each of the FDM channels carrying a 5.2 Mb/s data stream. Toprovide Boeing's IDS forward service links, each of the forward feederlink FDM data streams will be demodulated on board the satellite. Thedata stream will then be encoded and re-modulated as a code divisionmultiplex/time division multiplex (“CDM/TDM”) signal with a service linkbandwidth of 166.7 MHz and routed to the appropriate downlink servicetransmitters and beams. Depending on traffic requirements, more than one5.2 Mb/s information stream can be directed to a single beam. (FIG. 13)

[0101] The return links employ a transparent bent-pipe transponder. As aresult, the signal characteristics of the return service link and returnfeeder link are the same. The IDS return service link operates as a codedivision multiple access (“CDMA”) channel containing two differentsignal types. IDS return service link signals can include narrow band(4.8 Kb/s) data streams that are spread spectrum encoded to a bandwidthof 1.25 MHz. These narrow band signals are intended primarily for voice,fax and low rate data transmissions. IDS return service link signals canalso include wider band (76.8 Kb/s) data streams that are spreadspectrum encoded to a bandwidth of 20 MHz.

[0102] Boeing's return service link will employ power control from theuser terminal, primarily to compensate for rain fade in different partsof the return beam. The use of power control will maximize the number ofpermissible return channels by equally distributing the receivedinterference power between them.

[0103] 2.3 Backhaul Data Services (‘BDS’)

[0104] Each Boeing satellite will generate up to five steerable transmitbeams and five steerable receive beams. The transmit and receive beamswill be operated in pairs to provide two-way data transmission servicesto users at any point in a satellite's field-of-regard. The agile beamsfor BDS will be generated on each satellite using multi-beam transmitand receive phased array antennas. Pointing accuracy between userterminals and satellite receive antennas will be maintained usingautomatic tracking algorithms following ground-commanded initialacquisition. Phased array antennas transmitting BDS signals from eachsatellite will be slaved to the receive beams.

[0105] The forward links for Boeing's BDS will use a transparentbent-pipe transponder and, as a result, the signal characteristics ofthe forward service links will be the same as the forward feeder links.Both of the BDS forward links will employ TDM channels, each of whichwill carry a 24.4 Mb/s information-stream. The return links for BDS willalso employ a bent-pipe transponder. To provide flexibility for users,Boeing's BDS return links can operate using a signal design of either 60Mb/s quadrature phase shift keyed (“QPSK”) or 120 Mb/s 16 quadratureamplitude modulation (“16 QAM”).

[0106] 2.4 Ku-Band Spectrum Sharing

[0107] The Boeing system can operate co-frequency with the broadcastsatellite service (“BSS”), the GSO FSS, terrestrial FS networks, andother NGSO FSS systems. To protect GSO satellites, Boeing will employsatellite diversity using a ±15° latitude equatorial exclusion zone forforward and return service links. Before a Boeing satellite enters theexclusion zone, its traffic will be handed off to another Boeingsatellite that is outside of the exclusion zone. Boeing satellitesoperating outside of the exclusion zone will have large discriminationangles as viewed by GSO network earth station antennas.

[0108] 2.5 IDS Forward and Service Feeder Links

[0109] The IDS forward feeder link channels have a bandwidth of 6.25 MHzand are spaced at 7.15 MHz. A total of 122 channels are required for theforward feeder link. Both senses of circular polarization are used with61 channels on each polarization. TT&C channels are also provided on theforward feeder link at each end of the band. As a result, the bandwidthrequirement for the IDS forward feeder link is 450 MHz.

[0110] The IDS forward feeder link signal is de-multiplexed andde-modulated on-board the satellite and coded and re-modulated usingcode division multiplex (“CDM”) for transmission to user terminals. Eachspread spectrum CDM channel requires 166.7 MHz of bandwidth. Athree-cell frequency reuse pattern within a pattern of 37 spot-beamsprovides service to the user terminals. Both senses of circularpolarization and two frequency channels are used in each spot beam,resulting in a forward service link bandwidth requirement of 1000 MHz.

[0111] 2.6 IDS Return Service and Feeder Links

[0112] The IDS return service uses the same 37-spot-beam pattern andthree-cell frequency reuse as the forward service link. CDMA is used ina bent-pipe transponder architecture. Each CDMA channel employs abandwidth of 20 MHz. Two frequency channels and both senses of circularpolarization are used in each spot beam, resulting in a bandwidthrequirement of 120 MHz for the return service link.

[0113] Return service link channels are switched to the feeder linkchannels based on spot beam loading and, thus, are not one-to-one mappedto the return feeder link channels. Ninety eight return feeder linkchannels are provided 49 channels on each polarization. Additionally,two TT&C channels are provided for spacecraft operations, resulting in atotal return feeder link bandwidth requirement of 1000 MHz.

[0114] 2.7 BDS Links

[0115] The BDS forward feeder link will operate concurrently with theIDS forward feeder link on each satellite using separate gateways. Eachof the BDS forward feeder link channels has a bandwidth of 24.0 MHz anduses carrier spacing of 29.16 MHz. A total of five channels are requiredfor the forward feeder uplink using right-hand circular polarization(“RHCP”).

[0116] The BDS forward service link channels are frequency translatedfrom the forward feeder link frequencies with fixed offsets. The BDSforward service links will operate in the same spectrum as the IDSforward service links using geographic diversity to avoid intra-systeminterference.

[0117] The BDS return service link uses polarization reuse as shown inthe typical frequency band plan in FIG. 12. Each of the BDS returnchannels has a bandwidth of 58.7 MHz and uses carrier spacing of 68.5MHz.

[0118] BDS return service link channels are translated to the singlepolarized return feeder link channels with fixed offsets to the typicalfrequency plan.

[0119] 2.8 Beacon Link

[0120] Within the IDS forward service bandwidth, beacon signals are usedto allow the user terminals to acquire and synchronize with thesatellite transmissions before initiating service. Two beacon channelswill be provided. The first beacon channel will be centered at afrequency between the spot 1 and spot 2 downlink bands of the lowerportion of the IDS service downlink band. The second beacon channel willbe centered at a frequency between the spot 2 and spot 3 downlink bandsof the lower portion of the IDS service downlink band. TABLE 4 MEOConstellation Orbital Elements Number of Satellites 20 Number of OrbitalPlanes 4 Number of Satellites per Orbit Plane 5 Inclination of OrbitalPlanes (degrees) 57 Period of Orbit (hrs.) 11.97 Apogee of Orbit (km.)20,182 Perigee of Orbit (km.) 20,182 Argument of Perigee for each Orbit(deg.) 0 Eccentricity 0 Mean Anomaly of the i-th. Satellite (deg.) 0,72, 144, 216, 288, 36, 108, 180, 252, 324, 72, 144, 216, 288, 0, 108,180, 252, 324, 36 Right Ascension of the Ascending Node of 0, 0, 0, 0,0, 90, 90, 90, 90, the i-th. Satellite (deg.) 90, 180, 180, 180, 180,180, 270, 270, 270, 270, 270 Active Service Arc (deg.) 360*

[0121] 2.9 Details concerning the Satellites

[0122] The spacecraft antennas include two feeder link antennas, sevenforward service link antennas and one return service link antenna forthe IDS; along with transmit and receive multi-beam phased arrayantennas for the BDS and an Earth-coverage beacon antenna.

[0123] The spacecraft feeder link antennas consist of twofocal-point-fed parabolic antennas with steerable splash plates. Twoantennas are required in order to provide make-before-break connectivitywith the gateway stations. Each of the dual polarized one-meter diameterfeeder link antennas is illuminated by a dual-band waveguide feed. Thesplash plates are gimbaled about two axes to provide antenna-pointingcoverage over the Earth field-of-view.

[0124] The IDS forward service link antenna suite (FIGS. 13 & 14) usesseven symmetric 0.4-meter lens antennas. Taken together, these antennasprovide 37 dual polarized beams covering the spacecraft field-of-regard.One central antenna provides beams with both polarizations for thecentral seven beam positions. Each of the identical outer six antennasprovides beams for five outer beam positions using both polarizations.The boresights and rotations of all of the outer antennas are arrangedon the spacecraft nadir surface to properly align each antenna's beamswith respect to the aggregate.

[0125] A 37-beam direct radiating array is used to generate the IDSreturn service link beams. These beam patterns, each having RHCP andleft-hand circular polarization (LHCP), are designed to match theforward service link antenna beam coverage. As a result, the patternsfor the forward service link antenna are also applicable for thecorresponding beams of the IDS return service link antenna.

[0126] Separate transmit and receive multi-beam phased array antennasare used for the BDS service links. Each antenna can create up to fiveagile and independent beams. The transmit antennas will use RHCP beams.Two of the BDS receive antenna beams are RHCP, while the remaining threeare LHCP. Details concerning phased array antennas can be found in U.S.Pat. Nos. 4,939,527; 5,276,455; 5,488,380; 543,805; and 5,751,248, whichI incorporate by reference.

[0127] An Earth-coverage beacon is used to facilitate spacecraftacquisition and tracking by the user.

[0128] The growth of the “bandwidth on demand” (BOD) market is beingfueled by the convergence of data transmission carriage paths fornumerous telecommunications services. Many of these telecommunicationsservices are currently provided to consumers via separate lines oftransmission, but can be provided more economically using single datacarriage paths. For example, services such as basic voice telephony,facsimile transmission, video-conferencing, high-speed data transfer forlocal area networks (“LANs”), Internet access, and “push” type databroadcast services can be brought together during transmission usingstandardized data protocols.

[0129] Perhaps the greatest challenge in the cost-effective provision ofBOD is responding to dynamic variations in each customer's datatransmission needs. In the past, the provision of basic telephonyinvolved relatively constant and predictable bandwidth requirementsequally divided between the forward and return directions. BOD, however,necessitates constantly changing bandwidth capacity, often withdisproportionate forward and return throughput requirements. Forexample, corporations that have numerous, widely dispersed branchoffices may use private computer networks to collect sales informationfrom the field. These same companies also broadcast data and informationin the forward direction, addressing new products and services, trainingprocedures, computer software upgrades and other data-intensiveapplications.

[0130] Regardless of the specific use, professional users desireseamless interaction with communications networks. Professional usersprefer network communications systems that are capable of avoidingsignificant delays in data transmission rates, regardless of the amountof data transmitted and the direction employed. The most satisfactoryand cost effective method of accommodating this demand is using a datatransmission scheme that can dynamically adjust to allocate bandwidthbetween individual users—expanding to meet the momentary needs of onecustomer and subsequently contracting to free up capacity for others.This “flexible pipe” approach is the essence of Boeing's BODtransmission architecture.

[0131] In the forward direction, IDS will employ a “bit-container”transmission structure, permitting the provision of service to a largenumber of users through each gateway. The bit-container approach willalso enable Boeing to offer individual users alternativemodulation/error protection techniques within a bit-container. One typeof bit-container under consideration for the Boeing IDS system uses theMotion Picture Experts Group (“MPEG”) standard. Because of itswidespread use, reliable MPEG components and subsystems are available ona cost-effective basis. The MPEG approach also has many desirablefeatures for IDS, including the fact that input data streams can bemultiplexed without decoding/re-encoding.

[0132] On the return link, Boeing's IDS system will employ singlechannel per carrier techniques using CDMA, with a bent-pipe transponderon each satellite. Individual carriers are appropriate because thereturn link will include transmissions from users at relatively modestbit rates, making the use of a bit-container approach undesirable.Boeing's IDS system is designed to accommodate up to 96 simultaneoususers per beam (at 76.8 Kbps), with a total of 37 beams per polarizationper spacecraft.

[0133] The BDS is designed to support users that need to transfer largeamounts of data from a location in a short period of time. BDS will beable to provide high speed (up to 120 Mb/s) return link data transferfrom remote locations. At the same time, BDS will permit modest (24.4Mb/s) forward link capability. BDS will be provided using independentlysteerable beams and associated transmit/receivers, permitting up to fivesimultaneous users to be accommodated by each spacecraft. The userterminals for BDS will employ antennas with diameters in the vicinity ofone meter, making them useful for operation in remote locations.

[0134] The allocation of spectrum between Boeing's IDS and BDS systemswill be dictated by consumer demand for each service in a givensatellite footprint at particular times of the day. The flexibility andreliability of Boeing's dual BOD transmission schemes will be able toaccommodate a wide variety of professional, institutional andgovernmental uses. A sampling of potential applications include:

[0135] Corporate networking for dispersed corporate offices where anumber of people in individual in-house networks are tied together in awide area mesh network.

[0136] Banking and commercial transactions, where documents, contractsand databases need to be exchanged with substantial accuracy and in asecure mode.

[0137] Distance learning for corporate training, specialized educationand professional seminars.

[0138] Medical applications include the exchange of data, X-ray images,CAT scan data and EKG traces.

[0139] Publishing, where designers, artists and customers must exchangehigh-resolution color images - both fixed and moving.

[0140] Entertainment, where high-resolution audio and video materialmust be backhauled to a central production and redistribution facility.

[0141] Remote mining and exploration activities, where geologicalsampling data needs to be transmitted back to a central location foranalysis.

[0142] In each of these examples, users can be expected to havecustomized communication and data transfer needs, including diversecapacity, bit error rate and availability requirements. Some users willhave large transmission capabilities with modest error rates (e.g., 1 in105) for such applications as multiple high resolution X-rays anddistance learning. Other users will have high transmission reliability(e.g., low error rates down to 1 in 109) for conditional access,downloading of software, banking data and other information. In eachcase, the dynamic data transfer needs of Boeing's customers can beefficiently accommodated utilizing the BOD transmission capabilities ofBoeing's IDS and BDS transmission systems.

[0143] Boeing's phased array antenna steers beams electronically,permitting continuous connections between satellites and mobileplatforms. It measures two feet by three feet and is only about one inchthick, and can be embedded on the surface of aircraft and other mobileplatforms. The antenna is designed to acquire and maintain connectivitywith Boeing satellites using a beacon tracking technique similar to theapproach used in conical scanning. The tracking system can adjust tosudden shifts in relative source location, thus fully compensating forthe pitch and roll motions that are inherent in mobile platforms. Boeingterminals with phased array antennas cannot begin transmitting until thecorrect satellite signal has been acquired and is being tracked. In theevent that an antenna loses lock on the satellite beacon signal, alltransmissions would cease until the beacon signal has again beenacquired and the satellite is being tracked.

[0144] The satellite tracking and reception capabilities of Boeing'sphased array antenna have been tested on high speed mobile platforms,such as commercial and private aircraft. Over 2000 hours of operation,these systems have demonstrated that the antenna can track and maintainlock on a satellite signal during normal aircraft maneuvers. UsingBoeing's phased array antenna technology, Boeing can provide each of theelements of its ancillary mobile services without resulting inunacceptable interference to other users of the Ku-band.

[0145] The use of airborne transmitters raises certain co-frequencyinterference considerations not present with terrestrial-basedtransmitters. For example, an airborne transmitter affixed to themetallic fuselage of an aircraft can act as a radiator in all directionsand an airborne transmitter at altitude can radiate power for hundredsof miles.

[0146] The additional interference concerns relevant to airbornetransmitters, however, do not compromise Boeing's ability to protectco-frequency GSO systems. Boeing's ancillary transmissions from airborneplatforms will have interference characteristics that are within the ITUand FCC limits for NGSO FSS transmissions in the Ku-band. While theadditional interference concerns do raise an issue with respect toprotecting co-frequency terrestrial systems, Boeing has been unable toidentify any terrestrial services currently operating that would beaffected by Boeing's service.

[0147] Next, I will describe the signals of the Boeing satellite interms of the modulation, coding and multiple access schemes used for thevarious links in the system. To assist in describing the links, a briefdescription of the spacecraft communications payload is also included.Spare receivers, demodulators, power amplifiers and other relevantsubsystems are provided on-board the spacecraft and are activated in theevent of a failure. I will describe the basic operation of the systemand will not include a discussion of component sparing.

[0148] On each spacecraft, two sets of feeder link antennas, receiversand transmitters concurrently track two separate gateway locations.Tracking multiple gateways is required to provide a seamless handoff aseach satellite moves from the coverage area of one gateway to another.The IDS feeder link signals from the assigned gateways are demultiplexedand demodulated on-board the spacecraft. Each of the demodulated signalsfrom the active IDS gateway are switched to the appropriate downlinkbeam modulators, where they are re-encoded and re-modulated as spreadspectrum signals. The re-modulated signals are amplified and input tothe 37-beam transmit antenna. This process is shown in the block diagramof FIG. 13.

[0149] The signal characteristics for the IDS forward feeder link areprovided in Table 5. TABLE 5 Signal Characteristics for the IDS ForwardFeeder Link Channel Information Rate 5.2 Mb/s BER 10⁻⁴* FEC Coding 1/2rate SCCC** Modulation QPSK Spectrum Shaping SRC, β = 0.2 Necessary 6.24MHz Bandwidth Emission Designator 6M24G7W Multiplex FDM/TDM AvailabilityObjective 99.9%

[0150] The signal characteristics for the forward service link areprovided in Table 6. Multiple feeder link signals are code divisionmultiplexed onto a single downlink carrier using orthogonal codesequences for each different signal. In a typical CDMA system, signalsoriginate from multiple sources and therefore are not synchronized andorthogonal. As a result, each signal causes additional interference toeach of the other CDMA signals. This interference is sometimes referredto as the “cocktail party effect.” The more users there are in achannel, the louder (more power) they have to be to be heard by thereceiver. When all the signals are synchronized and orthogonal, however,there is no mutual interference and no power increase is necessary overthat needed for a single signal. This orthogonal signal structureminimizes the transmit power required and, therefore, minimizesinterference into co-frequency services. TABLE 6 Signal Characteristicsfor the IDS Forward Service Link Channel Information Rate 5.2 Mb/s BER10⁻⁹ FEC Coding 1/2 rate SCCC Spreading Ratio 24 Modulation QPSKSpectrum Shaping SRC, β = 0.33 Necessary 166.4 MHz Bandwidth Emissions166MG7W Designator Multiplex CDM/TDM Availability Objective 99.7%

[0151] A block diagram of the IDS return link processing is shown inFIG. 14. Signals on the return link are received from the user terminalsthrough the 37-beam receive antenna using either RHCP or LHCP. The 37receive channels on each polarization are routed via the switch to thefrequency translators for the appropriate feeder downlink channelassignment. The assignment of service link channels to feeder linkchannels will be done dynamically in the switch and will be at thediscretion of the ground control. The feeder link channels will then bemultiplexed into a single wideband signal, which will be amplified andtransmitted on the assigned feeder downlink antennas to the IDS gateway.

[0152] Two signal types have been defined for the IDS return link, a lowrate signal and a medium rate signal. The signal characteristics for theIDS low rate and medium rate return link signals are provided in Table7. Power control is used on the return service link primarily tocompensate for rain fade in different parts of the return service beam.TABLE 7 Signal Characteristics for the IDS Return Service Link ChannelLow rate Medium rate Information Rate 4.8 Kb/s 76.8 Kb/s BER 10⁻⁶ 10⁻⁶FEC Coding 1/3 rate, 1/3 rate, convolutional convolutional DataModulation 64-ary orthogonal 64-ary orthogonal Spreading rate 16 16 ChipRate 2.4576 Mc/s 39.3216 Mc/s Spreading QPSK QPSK Modulation Necessary1.25 MHz 20.0 MHz Bandwidth Emission Designator 1M25G7W 20M0G7W MultipleAccess CDMA CDMA Availability Objective 99.7% 99.7%

[0153] A list of the signal characteristics and the emission designatorsfor a typical BDS forward feeder and service link channel is given inTable 8. The link budget for a typical BDS forward feeder and servicelink channel (and for our other systems) is provided in Boeing's FCCApplication. A block diagram of the BDS payload and connectivity withthe feeder link antenna system is shown in FIG. 15. TABLE 8 SignalCharacteristics for the BDS Forward Feeder and Service Link InformationRate 24.4 Mb/s BER 10⁻¹⁰ FEC Coding Concatenated code, rate 0.614R-S(204,188) and rate 2/3 convolutional, K = 7 Modulation QPSK SpectrumShaping SRC, β = 0.2 Necessary Bandwidth 24.0 MHz Emission Designator24M0G7D

[0154] After carrier frequency translation, transmission of the BDSforward carriers is accomplished by the spacecraft transmit phased arrayantenna. This multi-beam antenna, with an effective area of one squaremeter, transmits multiple carriers using RHCP, with the total availableoutput power of 13 dBW shared between the active beams, as needed.

[0155] A list of the signal characteristics and the emission designatorsfor a typical channel are given in Table 9 or the return link. TABLE 9Signal Characteristics for the BDS Return Service and Feeder LinkInformation Rate 60 Mb/s 120 Mb/s BER 10⁻¹⁰ 10⁻¹⁰ FEC CodingConcatenated code, rate Concatenated code, rate 0.614 R-S(204,188) and0.614 R-S(204,188) and rate2/3 convolutional, rate2/3 convolutional, K =7 K = 7 Modulation QPSK 16QAM Spectrum Shaping SRC, β = 0.2 SRC, β = 0.2Necessary 58.7 MHz 58.7 MHz Bandwidth Emission Designator 58M7G7D58M7D7D

[0156] Reception of the BDS return service link carriers on thespacecraft is accomplished using a phased array antenna. The antennareceives multiple carriers using RHCP and LHCP. FIG. 16 shows the blockdiagram of the BDS return link payload.

[0157] The satellite transmits a beacon signal to provide the means foracquisition and tracking of the satellite by the user terminals. Table10 lists the signal characteristics of the beacon signal. TABLE 10Signal Characteristics of the Beacon Information Rate 1.0 Kb/s BER 10−⁶FEC Coding 1/2 rate convolutional Spreading Rate 256 Chip Rate 512 kc/sSpreading Modulation QPSK Spectral shaping SRC, b = 0.2 NecessaryBandwidth 307.2 kHz Emission Designator 307KG1D Availability Objective99.9%

[0158] In most cases, users will employ conventional reflector antennas.Some IDS users may find it advantageous to use Boeing's phased arraytransmit/receive antenna suite. Unless otherwise specified, thefollowing discussion applies to terminals with both conventional andphased array antennas.

[0159] Boeing system user terminals are divided into two segments: (1)the outdoor unit, which includes the antennas, associated RF components,connections to the indoor unit, and, where appropriate, antenna controlequipment and (2) the indoor unit, which provides the interfaces to theoutdoor unit and equipment that converts the signals to and fromuser-friendly interfaces such as switchboards, personal computers,instruments and telephones.

[0160] Each IDS user terminal will use two antennas. On a conventionalreflector-type installation, the two antennas will separately trackseparate satellites so that handoffs are seamless and unnoticeable bythe user. While one satellite is communicating with the terminal, theantenna controller can point the second antenna (or beam) in theexpected direction of a second satellite for handoff. After acquiringthe satellite beacon signal, the antenna (or beam) can begin closed looptracking of the satellite.

[0161] The user terminal will not make transmissions to the secondsatellite until after it has acquired the satellite beacon signal. Afterlink acquisition, dynamic power control is used on the return servicelink to compensate for rain-induced fades. Table 11 provides thetechnical characteristics of typical user terminals for the IDS.Antennas for the IDS user terminals will meet the sidelobe performancerequirements specified by: $\begin{matrix}{G = {29 - {25 \cdot {{Log}(\theta)}}}} & {{\left( {67 \cdot \frac{\lambda}{D}} \right){^\circ}} \leq \theta \leq 48^{{^\circ}}} \\{{- 10}\quad {dBi}} & {\theta \geq {48{^\circ}}}\end{matrix}$

TABLE 11 Typical IDS User Terminal Parameters Peak Antenna Gain 36.4 dBiTransmit Data Rate 76.8 Kb/s Receive Data Rate 5.2 Mb/s Max EIRP/channel45 dBW G/T (dB/K) 8.3 dB/K

[0162] BDS users will employ conventional reflector-type antennas. Table12 provides the technical characteristics of typical BDS user terminals.The BDS user terminals utilize a single reflector antenna which willmeet the performance requirements specified by: $\begin{matrix}{G = {29 - {25 \cdot {{Log}(\theta)}}}} & {\left( {100 \cdot \frac{\lambda}{D}} \right) \leq \theta \leq {2{^\circ}}} \\{{- 3.5}\quad {dBi}} & {{{20{^\circ}} \leq \theta \leq {26.3{^\circ}}},} \\{G = {32 - {25 \cdot {{Log}(\theta)}}}} & {{{26.3{^\circ}} \leq \theta \leq {48{^\circ}}},} \\{{- 10}\quad {dBi}} & {\theta \geq {48{^\circ}}}\end{matrix}$

TABLE 12 Typical BDS User Terminal Parameters Modulation QPSK 16QAMTransmit Antenna Gain 42.9 dBi 42.9 dBi Transmit Data Rate 60 Mb/s 120Mb/s Receive Data Rate 24.4 Mb/s 24.4 Mb/s Max EIRP 48.8 dBW 58.8 dBWG/T (dB/K) 18.8 dB/K 18.8 dB/K

[0163] The parameters of a typical gateway antenna are given in Table13. Antennas used for the gateway terminals will meet the performancerequirements of Section 25.209 of the FCC rules. The transmittedeffective isotropic radiated power (“EIRP”) from each gateway will meetthe requirements of ITU Radio Regulation S5.502, along with any futureITU and FCC limits. Dynamic power control will be used on the gatewayuplink to compensate for rain fades. TABLE 13 Typical GatewayCharacteristics Antenna Diameter (minimum) 4.5 m 4.5 m Services IDS BDSTransmitted Bit-rate 5.2 Mb/s 24.4 Mb/s (per channel) Number of TransmitChannels 122 5 Received information Bit-rate 4.8 or 76.8 Kb/s 60 or 120Mb/s (per channel) EIRP 68-85 dB W 68-85 dB W G/T (clear sky) 28.8 dB/K28.8 dB/K

[0164] The Telemetry, Tracking and Control (“TT&C”) function monitorsthe health and status of the spacecraft and alerts operators to anyanomalous conditions that may occur. Use of spare components,station-keeping maneuvers and other commands are accomplished by thisfunction.

[0165] Two operational TT&C modes are provided in the Boeing system.Normally, TT&C will operate through feeder link antennas. Duringtransfer orbit and in contingency mode, TT&C will operate with anomni-directional antenna. Selection of the operational mode will be madeby ground command. Sufficient transmitter power is available to providefor reliable operation using either the feeder link antennas or theomni-directional antenna. The primary telemetry data mode will be pulsecode modulation (“PCM”).

[0166] The satellites will be three-axis stabilized, oriented in asun-orbit configuration. This configuration always permits thetransmitted RF energy to be from the nadir side of the spacecraft, whilekeeping the solar arrays facing toward the sun. Three-axis stabilizationprovides for a stable platform that minimizes the amount ofcommunications beam pointing. This configuration and stabilityeffectively permits all of the satellite's IDS beams to be “dragged”across the surface of the Earth. The stable platform also provides areference for the electronically steered BDS beams.

[0167] Each satellite will be based upon a commercially available Boeingdesign or one of similar capabilities and characteristics if purchasedfrom another spacecraft vendor. The bus design will utilize a modulararchitecture to maximize integration flexibility, while minimizingintegration cycle time. Separate payload, bus and propulsion moduleswill be built-up and tested in parallel prior to mating.

[0168] The structural design of the Boeing satellite provides supportfor payload and spacecraft subsystems, radiators, and thermal conductorsfor on-orbit thermal control, stiffness to ensure antenna-pointingaccuracy and to reduce dynamic loading during the launch environment.The structural elements will consist of a mix of aluminum honeycomb andgraphite epoxy panels with a long heritage of proven space flight. Thestructural design will permit launch on a wide range of launch vehicles.

[0169] The electrical power subsystem provides a reliable (>0.9) sourceof power throughout the mission life of the satellite, allowingcontinuous operations through the nearly one-hour duration of maximumeclipse encountered in MEO. The electrical power subsystem uses flightproven components (e.g., GaAs solar cells and NiH batteries) to meetmission requirements.

[0170] The thermal control subsystem provides the capability to conductor to radiate heat to and from various areas of the satellite or to deepspace throughout the orbit. The subsystem uses heat pipes embedded inthe structural panels of the spacecraft, thermal isolators, heaters,optical coatings and multi-layer insulation blankets with knowndegradation properties. This thermal control subsystem design isflexible enough to control temperature through the use of multiple zonesthat permit thermal isolation of one zone from the others, if required.

[0171] The attitude control system will maintain the desired sun-orbitprofile of the spacecraft throughout the orbit and minimize pointing ofthe communications beams. Attitude control sub-system components consistof redundant zero momentum reaction wheels, inertial reference units andstar sensors. Momentum de-saturation is achieved through the use ofelectromagnets, while rotational and stabilization maneuvers areperformed using reaction control motors. Each of the components of thissubsystem has had demonstrated flight experience, which results in areliable subsystem design that meets mission requirements.

[0172] The propulsion system's primary requirement is to provide impulsetransfer to the final operational orbit, constellation maintenancestation-keeping maneuvers, rotational and stabilization attitude controlmaneuvers and transfer from the operational orbit to the disposal orbitat the end of the satellite's operational life. A proven nitrogentetraoxide/hydrazine system will be used for any required maneuvers ofthe satellite. The simplicity in the propulsion design also permits awide selection of launch vehicles, with propellant loading changes madewhere appropriate.

[0173] Other details of the system are in Boeing's FCC Application thatI incorporated by reference.

[0174] U.S. patnet application Ser. Nos. 09/672,378, filed Sep. 28,2000, and 09/728,605, filed Dec. 11, 2000, describe a system and methodfor managing the collective power transmitted from a plurality of mobiletransmitters (aircraft) to a GSO satellite in a BOD system of the natureof the present invention. The method limits the collective powerspectral density to less than a specified FCC threshold limit. Thesystem controls the operation of the several transmitters to mitigateagainst excessive interference reaching a nearby, non-target GSOsatellite. I incorporate the applications by reference.

[0175] While I have described preferred embodiments, those skilled inthe art will readily recognize alternatives, variations, andmodifications that might be made without departing from the inventiveconcept. Therefore, interpret the claims liberally with the support ofthe full range of equivalents known to those of ordinary skill basedupon this description. The examples are given to illustrate theinvention and not intended to limit it. Accordingly, limit the claimsonly as necessary in view of the pertinent prior art.

I claim:
 1. A method to reduce interference between a GSO satellite andits GSO ground station when operating an NGSO satellite or constellationof satellites orbiting at medium earth orbit (MEO), comprising the stepof: terminating all transmission signals generated on a particular NGSOsatellite or eminating from an NGSO ground station in communication withthe NGSO satellite when the NGSO satellite is in an exclusion zonedefined for a GSO ground station antenna.
 2. The method of claim 1wherein the exclusion zone is from 15° North to 15° South latitude.
 3. Asatellite communications system implementing the method of claim 1 . 4.A satellite constellation communication system, comprising: a pluralityof satellites forming a constellation orbiting the Earth in four sets,each set defining an orbital plane, each plane inclined about 57°relative to the equator, the satellites being at an altitude of mediumearth orbit (MEO)of about 20,000 km, the system providing truly globalcoverage while substantially minimizing the number of operationalsatellites required.
 5. The system of claim 4 wherein eash set includesat least one spare satellite.
 6. The system of claim 4 wherein theconstellation has the satellites spaced so that at least two operationalsatellites are in view to a user outside the Tropics at an elevationangle relative to the user at or above 30°
 7. The system of claim 4further comprising a plurality of gateways, each gateway being stationedat a fixed location on the Earth and functioning as a networkdistributor for communication signals to and from a user and theconstellation, wherein the satellite spacing in the constellation andthe gateway locatons have each satellite having visible in a field ofview for communication transmission at least two gateways at anylocation of the satellite in its orbit.
 8. A commmunication satelliteparticularly adapted for use in a MEO satellite constellation and systemfor full service, global comunication or communication (voice) and datasignals, comprising: (a) a platform; (b) at least two transceivers onthe platform for receiving and transmitting of the signals, eachtransceiver including Integrated Digital Service (IDS) and Backhaul DataService (BDS) for transmission, each transceiver also having a feederlink antenna and a tracking control system adapted for pointing andtracking the feeder link antenna at a selected gateway on the Earth,wherein seamless handoff between gateways occurs because each feederlink antenna has its own, dedicated tracking control system to alloweach antenna to point at a different, selected gateway.
 9. The satelliteof claim 8 wherein the BDS uses a multi-beam electronically steeredphased array antenna.
 10. The satellite of claim 8 wherein the IDSsystem is substantially as shown in FIG. 13 and 14 and wherein the BDSsystem is substanially as shown in FIG. 15 and
 16. 11. A method formitigating intra-system interference in a satellite communication systemhaving Integrated Digital Service (IDS) and Backhaul Data Service (3DS)operating at essentially a common frequency, comprising the steps of:steering independent transmit antennas on separate satellites in aconstellation, each antenna casting a footprint on the ground duringtransmission, so that a user in one footprint receives an IDS signalfrom a first satellite and a BDS signal from another satellite.
 12. In asatellite communication system having a plurality of satellites in amedium earth orbit (MEO) constellation transmitting a signal to a userin a broadcast footprint on the Earth, the method for minimizinginterfeence with or emissions to co-frequency competing communicationsystems, comprising the step of: transmitting only from the onesatellite in the constellation that provides the highest elevation anglefor the user while turning off other transmitters on other satellitesotherwise capable of transmitting to the same footprint to conservepower consumption of the system.